Download 7-4 Hearing and Equlibirium

Hearing and Equilibrium
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Parts of the Ear
external ear
 middle ear
 inner ear

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Parts of the Ear
inner
ear
external ear
middle
ear
Eustachian tube
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Relationship of Ear to Mouth
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External Ear
collects sound waves and passes them
inward into the external auditory canal
 Includes:
- auricle
- external auditory canal
- tympanic membrane

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Auricle
flap of elastic cartilage, flared like a
funnel
 covered with thick skin
 helix - top
 lobule - bottom, earlobe
 attached to head by ligaments and
muscles

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External Auditory Canal
curved tube about 1 inch long
 skin-lined
 near exterior opening are ceruminous
glands
- produce cerumen (wax) which helps to
trap foreign material
 lies in temporal bone
 extends from auricle to tympanic membrane

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Tympanic Membrane
thin, semitransparent membrane of fibrous
connective tissue
 lies between external auditory canal and
middle ear
 cone-shaped structure with apex directed
medially
 sound waves in auditory canal cause
pressure changes that produce eardrum
vibrations
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
Middle Ear
air-filled cavity in temporal bone
 epithelium-lined
 contains auditory ossicles
 extends from eardrum to thin, bony partition
with two membrane covered openings
called oval and round window
 connected to mouth by Eustachian tube

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Auditory Ossicles
transfer vibrations from eardrum to oval
window of internal ear
 maleus is attached to the internal
surface of eardrum at apex
 incus is the intermediate bone
 stapes is attached by ligaments to the
membranous oval window

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Auditory Ossicles
(cont.)
malleous vibrates with tympanic membrane;
passes vibration to incus
 incus causes stapes to vibrate on oval window
 oval window is pushed in and out, causing
motion in fluid within internal ear
 action activates receptor cells
 impulses travel to temporal lobe of cerebrum
and sound sensation results

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Inner Ear
complex series of interconnecting
chambers
 Includes:
- bony or osseous labyrinth
- membranous labyrinth

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Osseous Labyrinth
bony canal within the temporal bone
 lined with periosteum
 contains the fluid perilymph
- chemically similar to cerebrospinal fluid
 Divided into three areas:
- semicircular canals
- vestibule
- cochlea
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
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Membranous Labyrinth
surrounded by cerebrospinal fluid
 series of sacs following general shape of
osseous labyrinth
 lined with epithelium
 contains endolymph
- chemically similar to intracellular fluid
 portions within bony canals called semicircular
ducts communicate with utricle and vestibule

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Semicircular Canals
three canals; anterior, posterior, and
lateral
 each end enlarges into swelling called
ampula
 lie at right angles to each other
 contain receptors for equilibrium

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Vestibule
oval central portion of bony labyrinth
 contains two sacs called the utricle and
saccule
- connect to each other by small duct
 contains receptors for equilibrium

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Cochlea
(koklea = snail shell)
coil-shaped cavity
 anterior to vestibule
 makes almost three turns
 central bony core called modiolus
 contains thin, bony shelf which divides
cochlea into upper and lower
compartments and smaller cochlear duct
 contains receptors for hearing

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Three Compartments of Cochlea
scala vestibuli
 scala tympani
 cochlear duct (scala media)

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Scala Vestibuli
above bony partition
 ends at oval window
 filled with perilymph

Scala Tympani
below bony partition
 ends at round window
 contains perilymph

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Cochlear Duct
(scala media)
portion of membranous labyrinth
 separated from scala vestibuli by vestibular
membrane
 separated from scala tympani by basilar
membrane
 organ of Corti (spiral organ) located on
basilar membrane within cochlear duct
 tectoral membrane projects over and in
contact with hair cells of spiral organ
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
Organ of Corti
spiral organ
 lined with epithelial cells
- support cells and 16,000 hair cells
contain receptors for auditory sensations

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Hair Cells
Two types:
- inner
- outer
 processes at apical end extend into
endolymph of cochlear duct
 synapse with fibers of the cochlear branch
of the vestibulocochlear nerve (VIII)
 easily damaged by high intensity sounds

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Sound Waves
sound sensations are heard by vibrations
transmitted through the air
 result from alternate compression and
decompression of air molecules
 most audible sound vibrations to human
ears are frequencies between 1000 and
4000 Hertz
 entire audible range is 20 - 20,000 Hz

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Pitch
determined by frequency of vibrations
 the higher the frequency of vibrations,
the higher the pitch (musical high note)

Loudness
determined by amplitude of sound
waves
 measured in decibels (dB)

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Decibels of Sounds
silence
 rustling leaves
 normal conversation
 crowd noise
 vacuum cleaner
 pneumatic drill
 uncomfortable sound
 painful sound
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0 dB
15 dB
45 dB
60 dB
75 dB
90 dB
120 dB
140 dB
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Physiology of Hearing
auricle directs waves into external auditory
canal
 sound waves strike tympanic membrane
 alternate compression and decompression
of air cause membrane vibration
 movement of membrane depends on
intensity and frequency of sound waves
 low-frequency = slow membrane vibration
 high-frequency = rapid membrane vibration
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
Physiology of Hearing
(cont.)
malleus connects in central area of
tympanic membrane
 malleus vibrates, conducting vibration to
incus and then stapes
 stapes pushes membrane on oval
window in and out
 movement of oval window causes
waves in perilymph of cochlea

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Physiology of Hearing
(cont.)
inward movement of oval window
pushes on perilymph of scala vestibuli
to scala tympani to round window into
middle ear
 movement of perilymph exerts pressure
on vestibular membrane
 pressure in endolymph inside cochlea
increases and decreases

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Physiology of Hearing
(cont.)
pressure fluctuations move basilar
membrane causing hair cells of spiral organ
to move against tectorial membrane leading
to generation of nerve impulses in cochlear
nerve fibers
 pathway extends into medulla oblongata
through midbrain to thalamus and on to
temporal lobes of cerebrum for
interpretation

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Physiology of Equlibrium
when body movement occurs, organs detect
motion and aid in maintaining balance
 organs provide information on which way is
up or down
 Organs of equlibrium:
- utricle
- saccule
- semicircular ducts

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Two kinds of Equilibrium
static
- maintenance of posture in response to
changes in body orientation relative to the
ground
 dynamic
- maintenance of body position, mainly the
head, in response to sudden movements

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Deafness
conduction deafness
- impairment of structures that transmit
vibrations
- punctured eardrum, otitis media, wax
buildup
 nerve deafness
- degeneration of receptors
- damage to receptor cells

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Ménière’s Syndrome
labyrinth disorder
 characterized by fluctuating loss of
hearing, vertigo, and tinnitus
 caused by an increased volume of
endolymph causing enlargement of the
labyrinth
 disease of cranial nerve VIII

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